1. Field of the Invention
The present invention relates to a humidifying system for a fuel cell, the system humidifying a gas to be humidified and supplying the gas to the fuel cell. In particular, present invention relates a technique which is effective in improving efficiency of humidification.
2. Description of Related Art
A fuel cell having a solid polymer membrane as an electrolyte membrane, for example, comprises a stack of a plurality of single cells which are layered.
Each single cell comprises a solid polymer membrane which has ionic conduction and anode and cathode electrodes held on the membrane in contact therewith. Hydrogen is supplied to a path for a fuel gas which is formed as a groove on the side of the membrane which is in contact with the anode electrode, while air is supplied to a path for an oxidizer which is formed as a groove on the side of the membrane which is in contact with the cathode electrode, and then electric power is generated by the electrochemical reaction which occurs between the electrodes in each single cell.
At this time, in order to keep the efficiency of power generation high, the solid polymer membrane must be maintained to be saturated with water so as to ensure the function of the membrane as a proton (hydrogen ion) conductive electrolyte.
However, during generation of electric power, dehydration of the solid polymer membrane may proceed due to loss of water produced by the electrochemical reaction from the system.
Accordingly, in order to maintain good ionic conduction, moisture must be supplied to the solid polymer membrane.
For example, Japanese Unexamined Patent Application, First Publication (Kokai), No. Hei 8-273687 discloses a humidifying system for a fuel cell which enables moisture supply to the solid polymer membrane by allowing air, which is to be supplied to the path in the stack for an oxidizer, to pass through a water permeable humidifier in advance in order to humidify the air.
This humidifier has a structure similar to a multitubular heat exchanger, that is, a structure by which air can be allowed to flow inside hollow portions of a hollow fiber membrane bundle which is an assembly of hollow fiber membranes while water can be allowed to flow inside a jacket which houses the hollow fiber membrane bundle.
The hollow fiber membranes are characterized by permitting permeation of water while inhibiting permeation of gas. Thus, the hollow fiber membranes function as water exchange membranes by allowing water to penetrate through from a side on which the partial pressure of water vapor is high to a side on which the partial pressure of water vapor is low.
Accordingly, when water is allowed to flow inside the jacket, the water permeates through each hollow fiber membrane and diffuses inside the hollow portion of each hollow fiber membrane as water vapor, and thus the air which flows inside the hollow portion of each hollow fiber membrane can be humidified.
It should be noted that, in order to supply air to the humidifier and the fuel cell, a device for taking in outside air and conveying it by compression, such as a supercharger, is required.
FIG. 5 is a block diagram of a system showing a conventional example of humidifying system for a fuel cell. In the figure, reference numeral 101 indicates the fuel cell, 102 indicates a humidifier, 103 indicates an intercooler, 104 indicates a supercharger, and 105 indicates a pressure-regulating valve.
In this humidifying system for the fuel cell, the supercharger 104 is disposed upstream of the humidifier 102 in the direction of air flow from outside. The outside air is taken in by this supercharger 104 and is conveyed to the humidifier 102 by compression. Wet air obtained by humidifying the outside air in the humidifier 102 is supplied to the fuel cell 101.
However, the disposition of the supercharger 104 in the upstream of the humidifier 102 in the direction of air flow from outside requires a high pressure of air from outside to be supplied to the humidifier 102, and causes a problem that a high efficiency of humidification cannot be achieved. This is because of the following reason:
Since the driving force for the permeation of water is generated due to the difference in the partial pressure of water vapor, the efficiency of humidification can be improved by increasing the flow velocity of the air from outside rather than retaining the air from outside within the hollow portions of the hollow fiber membranes.
Accordingly, under the condition of a constant flow rate, when the air from outside under a high pressure is supplied into the hollow fiber membranes, the flow velocity of the air from outside which flows inside the hollow portions of the hollow fiber membranes is decreased, and the efficiency of humidification is also decreased.
This is also apparent from the pressure-dependent property of hollow fiber membranes (FIG. 6) and the water collecting property of hollow fiber membranes using air from outside (FIG. 7).
Accordingly, the conventional humidifying system for the fuel cell is defective in that the humidifier 102 and the supercharger 104 must be unavoidably large if achievement of a high efficiency of humidification is intended.
The present invention has been achieved in view of the above circumstances, and the object of the present invention is to improve the efficiency of humidification so as to be able to reduce the sizes of the humidifier and the supercharger.
In order to solve the above problems, the present invention employs the following constitution:
a humidifying system for a fuel cell, the system producing a wet gas (highly wet air Aw) by allowing a gas (dry air Ad), which is to be humidified, to pass through a water permeable humidifier (4), and supplying the wet gas to a fuel cell (1),
the humidifying system comprising a supercharger (3) between an inlet (47) for introducing the wet gas into the fuel cell (1) and an outlet (48) for releasing the wet gas from the humidifier.
Work L done by a supercharger is expressed by the following formula:
L=Gxc3x974.186xc3x97Cxc3x97(T+273.15)xc3x97(xcfx80c(kc-1)/kcxe2x88x921)xe2x80x83xe2x80x83(1) 
wherein
G: Flow rate (constant)
C: Specific heat of gas at inlet
T: Temperature of gas at inlet
kc: Ratio of specific heat of gas at inlet
xcfx80c: Compressor pressure ratio.
xcfx80c is given as follows by solving formula (1) for xcfx80c:
xcfx80c=(L/(Gxc3x974.186xc3x97Cxc3x97(T+273.15)))kc/(kc-1)xe2x80x83xe2x80x83(2) 
It is apparent from formula (2) that
xcfx80c greater than 1xe2x80x83xe2x80x83(3) 
On the other, since the following formula holds true:
xcfx80c=Pout/Pinxe2x80x83xe2x80x83(4) 
the relation between a pressure Pin of gas at the inlet and a pressure Pout of gas at the outlet is as follows from formulae (3) and (4):
Pressure Pout of gas at outlet
 greater than Pressure Pin of gas at inletxe2x80x83xe2x80x83(5) 
From formula (5), the following relation can be established when the flow rate G is constant:
Flow velocity Vin of gas at inlet
 greater than Flow velocity Vout of gas at outletxe2x80x83xe2x80x83(6) 
As described above, according to the present invention, since a supercharger is provided between an inlet for introducing gas into a fuel cell and an outlet for releasing gas from a humidifier, the flow velocity of the gas to be humidified which flows inside the humidifier can be increased in comparison with a system in which a supercharger is provided between an inlet for introducing gas into a humidifier and a supply source of the gas to be humidified, and thus the efficiency of humidification can be improved, and the sizes of the humidifier and the supercharger can be reduced.
In addition, the efficiency of supercharging can also be improved since a wet gas released from the humidifier can be supplied to the supercharger.